The diagram shows dark lines in the spectrum of light from the Sun, measured in a laboratory, and the same pattern of lines in the light from two distant galaxies, X and Y.
(a) Describe how the spectrum of galaxy X is different from the spectrum of the Sun. [1]
(b) What does this difference tell you about the motion of galaxy X? [1]
(c) What can you conclude about galaxy Y compared with galaxy X? Give a reason for your answer. [2]
- (a) the (pattern of) lines are shifted towards the red end (of the spectrum)
allow the lines are at longer wavelengths
ignore lines moved to the right unless linked to longer wavelength / red end(1) - (b) galaxy X is moving away from us (from the Earth / the Milky Way)
allow galaxy X is receding(1) - (c) galaxy Y is further away (from us) than galaxy X
allow galaxy Y is moving away faster than galaxy X(1) - (c) because the lines in Y are shifted further towards the red end / Y has a greater red-shift (than X)
a reason only scores if a comparison is given(1) - Answer: (a) lines shifted to longer wavelength (red end) (b) moving away (c) Y is further away / receding faster because it has a bigger red-shift
- Examiner tip: Say the lines move to longer wavelengths or towards the red end: "moved to the right" alone does not score.